1. Immunocytochemical techniques were applied for the first time in the common marmoset to locate the intracerebral sources of the neuropeptides oxytocin and vasopressin. Both vasopressin and oxytocin reactivity was found in cells within the paraventricular and supraoptic nuclei of the hypothalamus in adult males and females. 2. Under the influence of the anticholinergic drug benactyzine hydrochloride, castrated male squirrel monkeys exhibited alarm reactions of greater intensity than intact males, actually exceeding the reactivity levels of intact females. 3. Correlations between endogenous hormone levels and social behavior were found in newly paired common marmosets. Luteinizing hormone levels were positively correlated with incidence of a territorial marking vocalization, the twitter, in females. Scent marking, a nonvocal component of territorial marking, was negatively correlated with cortisol levels in both males and females. 4. A study of social cognition in adult squirrel monkeys demonstrated the ability to discriminate between familiar and unfamiliar individuals presented as videotaped images.